Power semiconductor modules require load current connections, which are passed to the outside, as an external interface for making electrical contact with the modules. In this case, depending on the power levels, external connections with relatively large cross sections may occur, and there is therefore a requirement for the load current connections to be highly robust. There is also a need to equip the load current connections with different conductor connecting contours, so that different cables or rail systems can be connected as required. On the side within the module, the load current connections must be electrically and mechanically connected to a less robust circuit mount, which is inserted in a housing, in general a plastic housing, and is fitted with components, in particular power semiconductors.
In this case, because of the high thermal conductivity and thermal coefficients of expansion of ceramic circuit mounts, so-called DCB (Direct Copper Bonded) substrates are preferably used for power semiconductors, which are composed of a ceramic insulator (aluminum oxide or aluminum nitride) on which a thin layer of pure copper is applied so that it adheres strongly. In particular, the high thermal conductivity together with the high thermal capacity and heat spreading of the copper coating and a coefficient of thermal expansion of the DCB substrate close to that of silicon make DCB substrates virtually indispensable for power electronics, despite their disadvantages in terms of robustness. When so-called naked semiconductor chips are mounted directly on the circuit mount, the electrical connection between the semiconductor chips and the circuit mount is normally made by bonding by means of thin Al or AG wires.
The electrical connection from the circuit mounted to the external load current connection is made using the same connecting technique. This admittedly results in a certain amount of mechanical decoupling between the load current connection and the circuit mount but, in principle, the different longitudinal coefficients of expansion of Al bonding wire (23*10−6) and DCB (7*10−7) result in restrictions in temperature life tests, since bonding wires may become detached. In any case, however, the introduction of large conductor connecting forces to the circuit mount should be avoided, and a permanent connection should be ensured between the circuit mount and the load current connection.
It is known from DE 197 19 703 for outer load current connections which project outwards to be injection molded in the module housing, with specific tabs in the load connecting conductor providing additional anchorage, since the shrinkage which occurs when the plastic housing cools down after the injection process means that the load connections are often not firmly anchored, at least not permanently. In this case, the connection between the circuit mount and the injection-molded load connections is made by bonding.
In this context, DE 199 14 741 likewise proposes load connecting elements which are connected to the inner wall of the housing and have bonding surfaces at their lower end. In this case as well, the connection between the respective load connecting element and the circuit mount is made by bonding wires. The types of connection described above both require bonding in the housing. An appropriate free space must be provided for the bonding tool in the housing for this purpose, and this has a disadvantageous effect on the module size, particularly in the case of small modules, since this space must be taken into account.
As mentioned, the circuit mounts are coated with copper, although, depending on the bonding wires that are used, surfaces (pads) which can be bonded and are composed of aluminum, nickel, copper or gold are required at the bonding connecting points for the bonding process, for which purpose the copper of the circuit mount is generally highly suitable. In some cases, the circuit mounts must, however, be provided with bonding pads. The load connecting elements themselves are in general produced in solid form from copper or brass and disadvantageously likewise require surfaces which can be bonded. Furthermore, depending on the current load, different bonding wire diameters are often required for DCB circuit connections between the components on the one hand and the circuit connection to the load connecting elements on the other hand. This involves an additional process step and cost disadvantages.
DE 36 43 288 describes various load connecting techniques. On the one hand, a current connecting bolt is described which is fitted to the circuit mount and is connected via a cross member and two screw connections to a mount body on the housing rim. A further connection from the mount body to the exterior is made possible by a separate screwed conductor element. Elsewhere, DE 36 43 288 describes a leaf spring element which is screwed on one side to a separate mount body, or a mount body which is integrated in the housing, and rests in a sprung manner on the circuit mount on the other side, making electrical contact. A further connection from the mount body located in the housing to the exterior is made possible by a separate conductor element, which is likewise screwed to the mount body. The large number of elements used and the need for multiple screw connections in these connecting techniques must have a disadvantageous effect on the production costs.
Another embodiment discloses a leaf spring element which has a cylindrical attachment, for engaging in the housing cover, on one side. When the housing cover is fitted, a contact pressure is exerted on the circuit mount. This may have the disadvantage that the contact is made only when the housing cover is closed. Furthermore, handling problems can be expected because of the fitting of the housing cover. A further disadvantage of this solution is that the area required on the circuit mount is comparatively large, and that the leaf spring occupies a comparatively large amount of space in the housing in this arrangement.
The latter could lead to space problems, and therefore to disadvantages, particularly when further discrete components of a higher type are fitted. Solutions based on leaf springs may be problematic overall in the event of incorrect sizes being used, especially when subject to increased shock and vibration loads.